SPERM FACTORS AND EGG ACTIVATION: Phospholipase C zeta (PLCZ1) and the clinical diagnosis of oocyte activation deficiency

Abstract

Oocyte activation deficiency (OAD) remains the predominant cause of total/low fertilization rate in assisted reproductive technology. Phospholipase C zeta (PLCZ1) is the dominant sperm-specific factor responsible for triggering oocyte activation in mammals. OAD has been linked to numerous PLCZ1 abnormalities in patients experiencing failed in vitro fertilization or intracytoplasmic sperm injection cycles. While significant efforts have enhanced our understanding of the clinical relevance of PLCZ1, and the potential effects of genetic variants upon functionality, our ability to apply PLCZ1 in a diagnostic or therapeutic role remains limited. Artificial oocyte activation is the only option for patients experiencing OAD but lacks a reliable diagnostic approach. Immunofluorescence analysis has revealed that the levels and localization patterns of PLCZ1 within sperm can help us to indirectly diagnose a patient's ability to induce oocyte activation. Screening of the gene encoding PLCZ1 protein is also critical if we are to fully determine the extent to which genetic factors might play a role in the aberrant expression and/or localization patterns observed in infertile patients. Collectively, these findings highlight the clinical potential of PLCZ1, both as a prognostic indicator of OAD and eventually as a therapeutic agent. In this review, we focus on our understanding of the association between OAD and PLCZ1 by discussing the localization and expression of this key protein in human sperm, the potential genetic causes of OAD, and the diagnostic tools that are currently available to us to identify PLCZ1 deficiency and select patients that would benefit from targeted therapy.

Figure 1

Representative immunofluorescence images of PLCZ1 localization in sperm from a fertile male. Images were captured as (A) bright-field or stained with (B) DAPI and (C) FITC-PLCZ1. Merged images are shown in (D). PLCZ1 is expressed in the equatorial (yellow arrows), acrosomal (white arrow), and postacrosomal (asterisk) regions in the sperm. Scale bar = 5 μm. Reproduced with permission from Meng et al. (2020).

Figure 2

Representative confocal images of PLCZ1 immunofluorescence in motile sperm organelle morphology examination (MSOME)-selected globozoospermic sperm exhibiting an acrosomal bud (red arrow). PLCZ1 (pale green staining) was localized to the midpiece (black arrowheads, A and B), or as a punctate pattern in the sperm head (white arrows, C), or in combination (D). White scale bar represents 5 µm. Reproduced with permission from Kashir et al. (2012b).

Figure 3

Artificial oocyte activation outcomes associated with two PLCZ1 parameters: mean PLCZ1 level in sperm (x-axis) and the proportion of sperm containing PLCZ1 (y-axis). ‘‘I, II, III, IV’ represent the four categories assigned by the two cutoffs. Patients with PLCZ1 deficiency are labelled in red (the red triangle: history with live birth with artificial oocyte activation (AOA); red dots: patients did not receive AOA; red star: patients received AOA but only chemical pregnancy). Patients who have had live births or ongoing pregnancy without AOA were labelled as green cubes, and patients with unknown outcomes of subsequent in vitro fertilization/intracytoplasmic sperm injection treatment were shown as black dots. For further details, see Meng et al. (2020). Reproduced with permission.

Figure 4

The suggested clinical algorithm for assessing the eligibility of a patient for artificial oocyte activation. AOA, artificial oocyte activation; ICSI, intracytoplasmic sperm injection. Reproduced with permission from Meng et al. (2020).